The present invention relates to a method for controlling a plasma processing apparatus and more specifically, it relates to a method for controlling the application of high frequency power in a parallel plates plasma processing apparatus.
So-called parallel plates plasma processing apparatuses achieved by providing an upper electrode (a first electrode) and a lower electrode (a second electrode) with a target object (substrate) mounted thereupon facing each other within a processing chamber constituted of an air-tight processing container have been proposed in the prior art. In a typical parallel plates plasma processing apparatus, a glow discharge is caused between the two electrodes by applying high frequency power for plasma generation to the upper electrode to generate plasma from the processing gas supplied to the processing chamber. By applying high frequency power for biasing to the lower electrode, ions in the plasma are drawn to the target object placed on the lower electrode to perform a specific type of plasma processing such as etching.
During the actual processing, first, a high frequency power of steady-state power, for instance, 27 MHz, for plasma generation is applied to the upper electrode at a time point t1, as indicated in FIG. 5(a), to generate plasma inside the processing chamber. Next, as FIG. 5(a) illustrates, after a specific length of time has elapsed and the plasma density inside the processing chamber has stabilized, i.e., at a time point t2, a high frequency power of steady-state power, for instance, 800 kHz, for biasing which is lower than the frequency of the high frequency power for plasma generation, is applied to the lower electrode so that the ions in the plasma are controlled to be drawn into the target object placed on the lower electrode.
Now, if high frequency power is applied between the two electrodes in the parallel plates plasma processing apparatus structured as described above, the impedance between the two electrodes is high until plasma is generated and an electrical current starts to flow between the electrodes, which results in a high voltage momentarily generated between the two electrodes.
However, since the frequency of the high frequency power for plasma generation is high, the impedance of the capacitor constituted by the two electrodes is not considered to be excessively high. As a result, the high voltage occurring between the two electrodes due to the application of the high frequency power for plasma generation is not high enough to present a significant problem.
In the case of the high frequency power for biasing whose frequency is relatively low, applied after plasma is generated inside the processing chamber and the impedance between the two electrodes becomes reduced, it is assumed in the prior art that it generates no high voltage such as that described above.
However, through observation made by the inventors of the present invention it has become clear that during actual processing, even when the high frequency power at a relatively low frequency for biasing is applied, a high voltage is generated between the two electrodes to result in, for instance, an abnormal discharge occurring at insulated portions inside the processing chamber.
Now, an explanation is given on the mechanism of such an abnormal discharge in reference to FIG. 5(b). First, when the high frequency power for plasma generation is applied to the upper electrode at the time point t1, the voltage rises abruptly at the moment of application. However, since the frequency of the high frequency power for plasma generation is high, the process and the apparatus are not adversely affected to the extent to which an abnormal discharge results.
Next, at the time point t2, at which it is assumed that the plasma inside the processing chamber has stabilized, the high frequency power for biasing is applied to the lower electrode. The application of the high frequency power for biasing destabilizes the plasma generation system that has been thus far stable. This temporarily reduces the plasma density to result in an increase in the impedance between the two electrodes. As a result, as indicated in FIG. 5(b), a high voltage is temporarily generated. When this happens, the peak of the voltage of high frequency power for plasma generation applied to the upper electrode is not high enough to affect the process or the apparatus as mentioned earlier. In contrast, the high voltage caused by the high frequency power for biasing applied to the lower electrode causes an abrupt increase in the voltage near the lower electrode which may cause an abnormal discharge inside the processing chamber to adversely affect the process and the apparatus to a considerable degree.
In addition, the high voltage occurring when the high frequency power for biasing is applied to the lower electrode causes a misalignment of the matching point of a matching device connected to the upper electrode. Also, because the matching device engages in a servo operation to correct such misalignment of the matching point, a delay occurs until the plasma inside the processing chamber becomes stabilized again.
Accordingly, a first object of the present invention, which has been completed by addressing the problems of the method for controlling a plasma processing apparatus in the prior art discussed above, is to provide a new and improved method for controlling a plasma processing apparatus that makes it possible to minimize the extent of the adverse effect on the process and the apparatus caused by the high voltage occurring when high frequency power for biasing is applied.
In addition, another object of the present invention is to provide a new and improved method for controlling a plasma processing apparatus that makes it possible to prevent an abnormal discharge which would occur when high frequency power for biasing is applied and to minimize the misalignment of the matching point for the upper electrode which would occur at the same time.
In order to achieve the objects described above, the present invention provides a method for controlling a plasma processing apparatus having a first electrode and a second electrode facing each other within the processing chamber and generates plasma inside the processing chamber by applying high frequency power for plasma generation having a first frequency to the first electrode via a first matching device and by applying high frequency power for biasing having a second frequency lower than the first frequency to the second electrode via a second matching device to perform a specific type of plasma processing on a target object placed on the second electrode.
In a first aspect of the present invention, the method for controlling the plasma processing apparatus described above comprises a step in which a high frequency power of steady-state power is applied to the first electrode and a high frequency power at a level at which, at least, the high frequency power for biasing can be matched, is applied to the second electrode and a step in which after the high frequency power for biasing has been substantially matched, the high frequency power applied to the second electrode is raised to the steady-state power level.
In this structure, first, plasma is generated inside the processing chamber by applying the high frequency power for plasma generation to the first electrode. In addition, high frequency power for biasing is applied to the second electrode. However, since the high frequency power applied to the second electrode is at a low level that allows the high frequency power for biasing to be substantially matched, its overshoot voltage is not high enough to cause an abnormal discharge even though it is in a low frequency range.
According to the knowledge of the inventors of the present invention, the matching point does not become greatly misaligned even if the power that is being applied changes once the plasma generation system has achieved a matched state, and the plasma generation system does not become overly destabilized even if the level of the high frequency power applied to the second electrode is raised to a steady-state after the plasma inside the processing chamber has been stabilized. Consequently, almost no high voltage resulting from an overshoot occurs as it would in the prior art.
In the first aspect of the present invention, an appropriate sensor, e.g., an optical sensor, is employed to verify that the high frequency power for biasing has been substantially matched and that the plasma has become stabilized before raising the high frequency power applied to the second electrode to the steady-state power level. In a second aspect of the present invention, data of a specific length of time is set into a recipe in advance and the high frequency power applied to the second electrode is raised to the steady-state level after the specific length of time has elapsed. Since this method allows the actual processing to be performed in conformance to a recipe determined in correspondence to optimal processing conditions using a dummy wafer or the like, simplification of processing is achieved. In addition, since devices such as the sensor for determining whether or not the high frequency power for biasing has been substantially matched can be omitted, the initial cost of the apparatus can be reduced.
Furthermore, while only the high frequency power for biasing applied to the second electrode is controlled in the first aspect and the second aspect of the present invention, control may be implemented for the high frequency power for plasma generation applied to the first electrode, as well. Namely, in a third aspect of the present invention, the method for controlling a plasma processing apparatus comprises a step in which high frequency power that enables, at least, plasma generation inside the processing chamber is applied to the first electrode and high frequency power at a level that enables, at least, the high frequency power for biasing to become matched is applied to the second electrode and a step in which after at least the high frequency power for biasing becomes substantially matched, the high frequency power applied to the first electrode and the high frequency power applied to the second electrode are raised to a steady-state level.
While the high frequency power applied to the first electrode is in a high frequency range that does not normally induce a high voltage resulting from a rapid overshoot, it may occasionally adversely affect the process or the apparatus with the high voltage occurring at the time of its application. Thus, by ensuring that the power applied to the first electrode is controlled at a level that is just sufficient to generate plasma inside the processing chamber as in the third aspect of the present invention, the extent to which the process and the apparatus are affected when the high frequency power is applied to the first electrode can be minimized in addition to achieving the advantages of the first aspect explained earlier.
In the third aspect of the present invention, too, the timing with which the high frequency power is raised to a steady-state level can be adjusted as in the second aspect of the present invention. Namely, in a fourth aspect of the present invention, the high frequency power applied to the first electrode and the high frequency power applied to the second electrode are raised to a steady steady-state level after the specific length of time has elapsed. This method achieves simplification both in the processing and the apparatus as in the second aspect of the present invention.
It is to be noted that in the first xcx9cfourth aspects of present invention, the power that allows matching applied to the second electrode during the initial stage of plasma generation should be at a level that does not allow etching to proceed, and in more specific terms, it is desirable to set the level of the power at roughly 3xcx9c10% of the steady-state power. In addition, in the third and fourth aspects of the present invention, it is desirable to set the level of the power applied to the first electrode for plasma generation during the initial stage of plasma generation at 50xcx9c70% of the steady-state power.
Moreover, in the third and fourth aspects of the present invention, it is desirable that when raising the high frequency power applied to the first electrode and the high frequency power applied to the second electrode to the steady-state power level, the power applied to the first electrode be set to the steady-state power level and then the power applied to the second electrode be set to the steady-state power level. By raising the power applied to the first electrode to the steady-state power level first, it is ensured that the impedance is reduced by increasing the plasma density between the two electrodes. Thus, when the power applied to the second electrode is subsequently raised to the steady-state power level, the resulting overshoot can be reduced.